Electric fuses whose voltage rating is relatively high call for fusible elements whose length exceeds that of the casing or fuse tube and which are wound substantially in the shape of a helix. In instances where the dimensional stability of helically wound fusible elements is relatively high, special supports of electric insulating material for the helically wound fusible elements may be dispensed with. Typical fuses of this nature are disclosed in U.S. Pat. No. 3,571,775 to F. J. Kozacka et al; 03/23/71 for HIGH-VOLTAGE FUSE HAVING A PLURALITY OF HELICALLY WOUND RIBBON FUSE LINKS. Many situations call for extremely thin fusible elements in ribbon form that have minimal dimensional stability. It is common practice in situations of this nature to provide mandrel-like supports of electric insulating material for the fusible elements around which the latter are wound helically. Typical fuses of this nature are disclosed in U.S. Pat. No. 3,680,019 to F. J. Kozacka; 07/25/72 for HIGH-VOLTAGE FUSE HAVING A PLURALITY OF FUSE LINKS WOUND HELICALLY AROUND AN INSULATING MANDREL. The presence of mandrel-like supports for helically wound fusible elements raises serious problems. All other conditions remaining unchanged, the presence of supporting mandrels for helically wound fusible elements drastically reduces the volume within the casing, or fuse tube, that is available for pulverulent arc-quenching filler. Another limitation resulting from the presence of mandrel-like supports for helically wound fusible elements are the cost of suitable mandrel materials. A desirable support material is high-grade alumina, the cost of which is high. The presence of mandrel-like supports for helically wound fusible elements gives rise to leakage current problems. Leakage currents may arise following a successful interruption of a faulted circuit by a fuse because the mandrel structure may become more or less conductive under the action of the arc. Mandrel-like supports for helically wound fusible elements which evolve gases when subjected to electric arcs contribute to successful interruption within given current ranges, but tend to result in other fault current ranges in the generation of very high internal pressures which impose unduly high demands on the dynamic bursting strength of casings, or fuse tubes.
U.S. Pat. No. 3,848,214 to E. Salzer; 11/12/74 for METHOD OF ASSEMBLING ELECTRIC HIGH-VOLTAGE FUSES AND SUBASSEMBLY THEREFOR discloses fuses having helically wound fusible elements that may be formed by narrow, extremely thin strips of metal, whose dimensional stability is minimal and which fuses do not require any mandrel-like structure of electric insulating material for the supporting of their fusible elements. The fuses disclosed in the above patent and their manufacture are predicated on the presence of plug terminals. This is an undesirable limitation in cases where the fuses are intended to be used in connection with fuse holders having contacts designed to cooperate with ferrules or terminal caps mounted on the outer surface of the casing or fuse tube.
The present invention relates to fuses having ferrules or terminal caps that are conductively interconnected by helically wound fusible elements, which elements are not required to be dimensionally stable, and not required to be supported by a mandrel-like supporting structure of electric insulating material.